Chronic rheumatoid arthritis is a systemic chronic inflammatory disease in which abnormal growth of connective tissue, including synovial tissue, occurs in the joints (Melnyk et al., Arthritis Rheum. 33: 493-500, 1990). The joints of chronic rheumatoid arthritis patients have been shown to have marked growth of synovial cells, formation of a multilayer structure due to abnormal growth of the synovial cells (pannus formation), invasion of the synovial cells into cartilage tissue and bone tissue, vascularization toward the synovial tissue, and infiltration of inflammatory cells such as lymphocytes and macrophages. Mechanisms of onset of chronic rheumatoid arthritis have been reported to be based on such factors as heredity, bacterial infection and the contribution of various cytokines and growth factors, but the overall mechanism of onset has remained unclear.
In recent years, cytokines and growth factors including interleukin-1 (IL-1), interleukin-8 (IL-8), tumor necrosis factor .alpha. (TNF.alpha.), transforming growth factor .beta. (TGF.beta.), fibroblast growth factor (FGF) and platelet-derived growth factor (PDGF) have been detected in the synovial membrane and synovial fluid of chronic rheumatoid arthritis patients (Nouri et al., Clin. Exp. Immunol. 55:295-302, 1984; Thornton et al., Clin. Exp. Immunol. 86:79-86, 1991; Saxne, et al., Arthritis Rheum. 31:1041-1045, 1988; Seitz et al., J. Clin. Invest. 87:463-469, 1991; Lafyatis et al., J. Immunol. 143:1142-1148, 1989; Melnyk et al., Arthritis Rheum. 33:493-500, 1990).
It is believed that IL-1, TNFA and PDGF are particularly powerful synovial cell growth factors (Thornton et al., Clin. Exp. Immunol. 86:79-86, 1991; Lafyatis et al., J. Immunol. 143:1142-1148, 1989; Gitter et al., Immunology 66:196-200, 1989). It has also been suggested that stimulation by IL-1 and TNF results in production of interleukin-6 (IL-6) by synovial cells (Ito et al., Arthritis Rheum. 35:1197-1201, 1992).
IL-6 is a cytokine also known as B cell-stimulating factor 2 or interferon .beta.2. IL-6 was discovered as a differentiation factor contributing to activation of B lymphoid cells (Hirano, T. et al., Nature 324, 73-76, 1986), and was later found to be a multifunction cytokine which influences the functioning of a variety of different cell types (Akira, S. et al., Adv. in Immunology 54, 1-78, 1993). Two functionally different membrane molecules are necessary for the induction of IL-6 activities. One of those is IL-6 receptor (IL-6R), an approximately 80 KD molecular weight, which binds specifically to IL-6.
IL-6R exists in a membrane-binding form which is expressed on the cell membrane and penetrates the cell membrane, as well as in the form of soluble IL-6R (sIL-6R) which consists mainly of the extracellular domain. Another protein is gp130 with a molecular weight of approximately 130 KD, which is non-ligand-binding but rather functions to mediate signal transduction. IL-6 and IL-6R form the complex IL-6/IL-6R which in turn binds with another membrane protein gp130, to induce the biological activity of IL-6 to the cell (Taga et al., J. Exp. Med. 196:967, 1987).
It has been reported that the serum or synovial fluid of chronic rheumatoid arthritis patients contains excessive amounts of interleukin-6 (IL-6) and soluble IL-6 receptor (sIL-6R) (Houssiau et al., Arthritis Rheum. 31:784-788, 1988; Hirano et al., Eur. J. Immunol. 18:1797-1801, 1988; Yoshioka et al., Japn. J. Rheumatol. in press), and since similar results have also been obtained in rheumatoid arthritis animal models (Takai et al., Arthritis Rheum. 32:594-600, 1989; Leisten et al. Clin. Immunol. Immunopathol. 56: 108-115, 1990), it has been suggested that IL-6 is somehow involved in chronic rheumatoid arthritis.
However, Japanese Unexamined Patent Publication No. 4-89433 discloses that peptides which strongly promote IL-6 production are effective as therapies for chronic rheumatoid arthritis.
Also, Higaki et al. have suggested that synovial cells from chronic rheumatoid arthritis patients have a low growth reaction against IL-6, and that IL-6 thus has an inhibitory function against growth of synovial cells (Clinical Immunology, 22:880-887, 1990). Thus, conflicting reports exist regarding the relationship between IL-6 and chronic rheumatoid arthritis, and the relationship is as yet unclear.
Recently, Wendling et al. have reported that administration of anti-IL-6 antibodies to chronic rheumatoid arthritis patients temporarily alleviates the clinical and biological symptoms, while also increasing IL-6 levels in the serum (J. Rheumatol. 20:259-262, 1993).
These reports provide no data at all about whether IL-6 accelerates growth of chronic rheumatoid arthritis synovial cells or has an inhibitory effect, and thus it is still unknown whether or not IL-6 has a direct effect on synovial cells of chronic rheumatoid arthritis patients.